Conventionally, in the manufacture of an electronic circuit board, for example, solder paste is generally used for soldering electronic components such as chip components, or the like, onto a printed circuit board, and a solder paste printing device is used in order to apply this solder paste by printing in a desired pattern.
solder paste printing by means of a solder paste printing device of this kind is now described on the basis of FIG. 4 and FIG. 5.
FIG. 4 is an approximate view of solder paste printing, and FIG. 5 is an expanded view thereof. In the drawings, 1 is a metal stencil wherein a desired pattern opening 2 is formed, and 3 is a printing squeegee which moves linearly in the direction of printing, 4 is a printed circuit board, 5 is a solder resist and 6 is a land whereon solder paste 7 is printed.
In the aforementioned composition, solder paste printing involves positioning a stencil 1 over the printed circuit board 4 and superimposing it thereupon, moving the squeegee 3 linearly along the direction of printing in a state whereby it is caused to contact the stencil 1 under a suitable pressure, filling solder paste 7 into the pattern opening 2 on the stencil, and then separating the printed circuit board 4 from the stencil 1, whereby solder paste 7 is applied by printing onto the land 6 of the printed circuit board 4 via the stencil 1.
Usually, the squeegee 3 is held by a squeegee head, and the composition of a squeegee head mounted in a conventional solder paste printing device is shown in FIGS. 6A and 6B, in which 8 is a micrometer for adjusting parallelism and 9 is a fixing bolt.
When conducting solder paste printing, firstly, the parallelism between the surface to be printed and the squeegee 3 is adjusted. After the squeegee has been adjusted so that it is parallel with the surface to be printed using the micrometer 8 for adjusting parallelism, it is fixed in this state using the fixing bolt 9. In this way, conventional solder paste printing is conducted by moving the squeegee 3 in its fixed state over the stencil 1.
The printed circuit board 4 forming the surface to be printed is usually not completely flat, but rather contains warping or undulation. Furthermore, in the case of double-sided component mounting, or the like, where electronic components are soldered to both the front and rear sides of the printed circuit board 4, there is a tendency for warping or undulation to be increased by heating during application of solder to the surface of the printed circuit board.
A case where solder paste is printed onto a surface to be printed containing warping or undulation in this way by means of a printing device of the aforementioned conventional composition is now described on the basis of FIG. 7. (FIG. 7 is an approximate diagram showing the state of a squeegee 3 on a surface 10 to be printed containing warping and undulation, and the stencil 1 is omitted from the drawing.)
In FIG. 7, since the squeegee 3 moves over the surface 10 to be printed in a fixed state, at recess sections 10a in the surface 10 to be printed the end section of the squeegee 3 is in a floating state, and conversely, at projecting section 10b, the deformation of the squeegee 3 increases and it presses more strongly than usual against the surface 10 to be printed. In fact, at the recess sections, the squeegee 3 does not wipe the solder paste 7 away satisfactorily, solder paste 7 remains on the stencil 1 and it is not filled completely into the pattern opening 2, whereas at the projecting sections, solder paste 7 filled into the pattern opening 2 is scraped away due to the increase in the deformation of the end sections of the squeegee 3, and hence there is a disparity in the amount of solder printed onto the surface 10 to be printed, and it is difficult to print a uniform amount of solder across the whole surface 10 to be printed.
Furthermore, since the adjustment of the parallelism of the squeegee prior to printing is a sensitive operation, individual differences between operators readily occur, leading to large variations in this adjustment, and the adjustment operation also takes time and causes productivity to fall.
Various means have been investigated with the object of resolving the aforementioned problems, for example, in a squeegee head of the composition shown in FIGS. 8A and 8B, the squeegee 3 is devised such that it is able to follow warping and undulation in the surface 10 to be printed, and in order to dispense with the need for adjustment of the parallelism of the squeegee, a lower squeegee holding member 21 for holding the squeegee 3 is coupled by means of a shaft 22 at a central position on the squeegee holding member 21 to a middle squeegee holding member or vertically moving member 24 which can move vertically along guides 23, as shown in FIGS. 8A and 8B, and pressure is applied to both end sections of the squeegee holding member 21 by air cylinders 11. Since the squeegee holding member 21 is rotatable in direction A as shown in FIG. 8A using the shaft 22 as an axis, rather than being fixed as in the prior art, the squeegee 3 can be rotated and moved vertically by applying pressure by means of air cylinders 11 provided at the end sections of the squeegee 3 centerd about a rotating axis X in line with the direction of travel of the squeegee.
According to this composition, as shown in FIGS. 9 and 10, by applying pressure to the squeegee 3 by means of the air cylinders 11 such that an appropriate printing pressure is achieved, the squeegee 3 rotates and moves vertically in response to recesses 10a and projections 10b in the surface 10 to be printed, in other words, the stroke of the air cylinders 11 varies, thereby enabling the squeegee 3 to follow the surface 10 to be printed at a constant printing pressure.
However, as shown in FIGS. 9 and 10, whilst the aforementioned composition is effective when the warping and undulation (recesses 10a and projections 10b) in the surface 10 to be printed are parallel to the longitudinal direction of the squeegee 3, in reality, the warping and undulation on a printed circuit board is complex and in general, little of it is parallel to the squeegee 3. Therefore, as shown in FIGS. 11 and 12, if it is not parallel, then similarly to a squeegee head of the aforementioned prior art composition, the squeegee 3 will assume a floating state above the surface 10 to be printed, there will be a disparity in the amount of solder printed onto the surface 10 to be printed, and it will be difficult to print a uniform amount of solder.